AGU journal highlights -- Nov. 6, 2006

1. Land surface evaporation increased during the second half of the 20th century

Evaporation from pans has been decreasing over many areas of the world for the past half century, but the significance of this trend is under debate. Though some speculate that decreases in pan evaporation result from well-documented "global dimming," where less solar irradiance reaches the ground, others hypothesize a complementary relationship between pan evaporation and actual evaporation. For example, in arid climates, terrestrial evaporation is low. However, water in pans left out in this environment can evaporate huge amounts of water. By contrast, water in pans left out in a more humid environment due to increased precipitation will tend to lose less water because of the ambient humidity. Wilfried Brutsaert shows through a mathematical model how lower pan evaporation rates actually indicate higher terrestrial evaporation, in spite of global dimming. Thus, while global dimming had an effect, it was not strong enough to cause a negative trend in evaporation where pan evaporation had been observed to decrease. Based on this, he suggests that the hydrologic cycle is accelerating in those areas.

Title:
Indications of increasing land surface evaporation during the second half of the 20th century

The average rate of Earth's geomagnetic reversals has varied enormously, from five reversals per million years during the last 10 to 20 million years to as low as 0.05 reversals every million years between 125 and 84 million years ago. Coe and Glatzmaier analyzed computer simulations of the geodynamo, the process by which magnetic field is produced by the Earth's convecting core. They found that geodynamos that produced highly equatorially symmetric fields were much less stable than those that produced highly asymmetric fields, consistent with earlier studies based on the paleomagnetism recorded by volcanic rocks. Further, a simulation with a solid inner core much smaller than today's produced a very asymmetric field, suggesting that reversals were much less common in the distant geologic past than in the more recent past. Though not definitive, because of the paucity of suitable continuous sections, the authors' review of paleomagnetic results from ancient rocks offers independent support for this conjecture.

Arenal, a small stratovolcano in Costa Rica, is currently experiencing activity characterized by continuous lava extrusion, frequent pyroclastic flows, and small ash emissions from its active vents. In 1998 and 2005, NASA's Laser Vegetation Imaging Sensor (LVIS), an airborne laser altimeter system, collected three-dimensional topographic images of the volcano. By recording the shape of reflected laser pulses, LVIS provides views of the vertical structure of Earth's surface, including both ground and canopy-top topographies. Hofton et al. compared data from 1998 with data from 2005 in order to relate changes in ground topography to recent disturbances by lava and pyroclastic flows. By mapping the flows deposited between these years, they found that the active crater grew by about four meters [10 feet] each year. They also estimate that materials extruded by the volcano during this time period reached about 20 million cubic meters700 million cubic feet]. The authors expect that similarly precise elevation change data will be essential to evaluating future hazards and risks at Arenal and other volcanoes.

On 8 October 2005, a large earthquake (Magnitude 7.6) shook northern Pakistan, causing more than 80,000 deaths. This earthquake primarily involved thrust motion on a northeast-dipping fault, according to rapid estimates made available a few hours after shaking ceased. Within a few days, Pathier et al. provided a more precise fault location based on synthetic aperture radar data collected from the European Space Agency's Environmental Satellite (ENVISAT). In this paper, the authors present a more extensive analysis of surface deformation, including detailed three-dimensional surface displacement maps of the entire epicenteral area and slip distribution inversions to model the earthquake mechanisms. In agreement with other studies, they found that slip occurred shallowly, that the upthrusted segments exhibited some transverse motion, and that the location of rupture initiation or arrest occurred at intersecting faults or other geomorphic features, implying structural control of the slip distribution. The authors expect that similar remote sensing analyses soon after disasters will be critical to directing post-disaster scientific investigations and to relief efforts, because such data can be used to help estimate damage.

Title:
Displacement field and slip distribution of the 2005 Kashmir earthquake from SAR imagery

Authors:
E. Pathier, T. J. Wright, R. Walker, and B. E. Parsons: Centre for the Observation and Modelling of Earthquakes and Tectonics (COMET), Department of Earth Sciences, University of Oxford, Oxford, United Kingdom;

5. Surface temperatures in China will increase despite a decrease in insolation

During the latter half of the 20th century, China experienced an increase in surface temperature, despite a decrease in insolation, which is the incoming solar radiation that reaches the surface. Rangwala et al. used observational data and global climate model simulations to examine trends in several climate variables, including surface insolation, surface air temperature, cloud cover, surface vapor and air pressure, and evaporation. Based on the model, the downward trend in insolation is expected to continue as more sulfate pollutants and other manmade aerosols, which scatter incoming solar radiation, are released by industries. However, surface temperatures are nonetheless modeled to increase. The authors suggest that both past and future warming are linked with an increase in downward longwave radiation, which is the radiation directed towards the surface that is emitted by the atmosphere itself. This increase in longwave radiation occurs partly in response to water vapor feedbacks triggered by the increase in manmade greenhouse gases that warm the surface.

Title:
Analysis of global climate model experiments to elucidate past and future changes in surface insolation and warming in China

Because earthquake shock sometimes decreases the frequency at which the ground vibrates, especially at soft soil sites, the strength of strong ground motion during an earthquake is an important characteristic that can affect the degree to which geological structures and buildings collapse. Although most research focuses on the reduction in peak frequency of site response caused by strong ground motion, Sawazaki, et al. sought instead to document peak frequency recovery following large earthquakes. By analyzing coda waves, which are seismic wave trains generated by energy scattering due to soil and crustal heterogeneities, the authors estimated peak frequency recovery using data from seismometers vertically separated in boreholes at two sites following Japan's 2000 Western Tottori Earthquake, and at a site following the 2003 Tokachi-Oki Earthquake. They find that the peak frequency at relatively solid sites after the Western Tottori Earthquake took a few years to recover to the value before the earthquake, but that the weak site after the Tokachi-Oki earthquake recovered within an hour.

Imaging techniques using seismic wave travel times are helpful in determining spatial structure, with applications ranging from medicine to hydrocarbon exploration. However, in cases such as volcano monitoring, temporal changes in seismic wave velocities at the subsurface are also of interest. Sens-Schönfelder and Wegler proposed a technique named Passive Image Interferometry to continuously monitor such changes. In this technique, seismic noise from two permanent seismic stations on the volcano are used to retrieve impulse responses (Green's functions) between the two stations. Using a case study of Indonesia's highly active Merapi volcano, the authors showed that by correlating seismic noise received by two stations, scattered waves from impulse responses could be retrieved. With this information, they inferred seismic velocity variations at Merapi. By tabulating these variations each day, the authors discovered a strong seasonal pattern, which they modeled to result from groundwater-level changes caused by the interplay of precipitation and drainage. This suggests that hydrological factors, rather than internal stress changes, greatly influence the shallow seismic velocity structure of the volcano during periods of volcanic quiescence.

8. A new technique for measuring turbulence dissipation rates in the ocean

Vertical exchange driven by turbulent mixing is important in determining momentum flux, heat flux, and material transport pathways in the ocean. A recent development in the measurement by Doppler acoustics of the rate of production of turbulent kinetic energy has improved understanding of vertical exchange processes, but is limited in spatial scales and requires an accurately vertical sensor platform. To overcome these limitations, Wiles et al. developed a new technique for measuring turbulent kinetic energy dissipation in the ocean by adapting methods from radar meteorology used to measure turbulence in the atmosphere. Using a standard acoustic Doppler current profiler, a type of sonar that records water current velocities over a range of depths, the authors related spatial correlations in velocity to turbulent kinetic energy dissipation rates. They then compared their estimates with estimates collected using more widely-used, but labor intensive, methods, and found good agreement. Advantages to their technique include its ability to provide long-term continuous time series of dissipation profiles, and that it can be used from a moving platform.

Title:
A novel technique for measuring the rate of turbulent dissipation in the marine environment

I. Highlights, including authors and their institutions
II. Ordering information for science writers and general public

I. Highlights, including authors and their institutions

The following highlights summarize research papers in Geophysical Research Letters (GL).

You may read the scientific abstract for any of these papers by
going to http://www.agu.org/pubs/search_options.shtml and
inserting into the search engine the portion of the doi (digital object
identifier) following 10.1029/ (e.g., 2005GL987654). The doi is
found at the end of each Highlight, below. To obtain the full text of
the research paper, see Part II.

II. Ordering information for science writers and general public

Journalists and public information officers of educational and
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The papers will be e-mailed as pdf attachments.

Others may purchase a copy of the paper online for nine dollars:

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Last reviewed:
By John M. Grohol, Psy.D. on
21 Feb 2009
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